Process for preparing hexagonal system prismatic calcium sulfite

ABSTRACT

HEXAGONAL SYSTEM PRISMATIC CALCIUM SULFITE, WHICH IS USEFUL AS A FILLER FOR THERMOPLASTIC RESINS, IN PREPARED BY CONTACTIG AN SO2-CONTAINING GAS WITH A SUSPENSION OF CALCIUM HYDROXIDE, PRECIPITATED CALCIUM CARBONATE, OR MIXTURES THEREOF WHEREIN THE SO2-CONTAINING GAS IS PASSED CONTINUOUSLY THROUGH A PLURALITY OF CONTACT ZONES IN WHICH THE FIRST OF SAID ZONES HAS A TEMPERATURE OF 50*-100*C. AND A PH OF 3-6 AND EACH SUCCEEDING CONTACT ZONE HAS A TEMPERATURE OF 50*-100*C. AND A PH OF HIGHER THAN 6, WHILE SIMULTANEOUSLY PASSING THE SUSPENSION THROUGH SAID PLURALITY OF CONTACT ZONES COUNTER-CURRENTLY TO THE DIRECTION OF THE FLOW OF THE SO2-CONTAINING GAS.

July 23, 1974 KEHCHl MURAKAMI ETAL `3,825,656

PROCESS FOR PREPARING HEXAGONAL SYSTEM PRISMATIC CALCIUM SULFITE Filed D60. 16, 1971 9 BYN www@ Mi/#MM ATTORNEY United 'States Patent' Oj'g 3,825,656 Patented July 23, 1974 PROCESS FOR PREPARING HEXAGONAL SYSTEM U.s. ci. 423-512 s claims ABSTRACT OF THE DISCLOSURE Hexagonal system prismatic calcium sulte, which is useful as a ller for thermoplastic resins, is prepared by contacting an SO2-containing gas with a suspension of calcium hydroxide, precipitated calcium carbonate, or mixtures thereof wherein the SO2-containing gas is passed continuously through a plurality of contact zones in which the first of said zones has a temperature of 50100 C. and a pI-I of 3-6 and each succeeding contact zone has a temperature of SOP-100 C. and a pH of higher than 6, while simultaneously passing the suspension through said plurality of contact zones counter-currently to the direction of the flow of the SO2-containing gas.

BACKGROUND OF THE INVENTION Field of the Invention This invention .relates generally to the formation of hexagonal system prismatic calcium sulte which may be advantageously used as a `filler material for thermoplastic resins.

Description of Prior Art Calcium suliite is often used to increase the rigidity of thermoplastic resin sheet material, particularly hi-gh density polyethylene sheet. However, its eiectiveness for this purpose has not been as high as anticipated, particularly because the conventional techniques for preparing calcium sulte usually results in a iine crystalline form having a particle size of less than 3a. Conventionally calcium sulte has been prepared by feeding SO,l gas in concentrations of less than 1%, into milk of lime. However, at these low gas concentrations, it has been quite diicult to obtain a uniform rate of SO2 feed, and the resulting crystals of CaSO3-1/2H2O are usually less than 3p. in particle size.

Although it has been possible to obtain calcium sulte in particle sizes of greater than 3a, these large particle sizes have only been obtainable by flocculation methods. Flocculation methods, however, do not result in the formation of a prismatic single crystal form of calcium sulte, which has been found tobe necessary for successful use as a filler.

When Ca(OH)2 is reacted with SO2, the reaction will be terminated while the reaction mixture is still in an alkaline condition, so that the degree of productivity of calcium suliite is quite low. The calcium sulfite prepared by that method is amorphous and usually has a particle size of about 3p.. If the reaction is continued into the acidic region, these small particles will be converted into the colloidal form, since the solubility of small particle size calcium suliite is quite high in an acidic solution of sulurous acid.

It has now been discovered'that the usefulness of calcium sulfite as a thermoplastic resin lfiller will be substantially improved if it is used in the form of hexagonal system prismatic crystals having a particle size of more than 3p, and particularly more than 5p..

` SUMMARY OF THE INVENTION Accordingly, it is one object of this invention to provide a `process for preparing hexagonal system prismatic crystals of calcium sulte, which can be used as a filler for thermoplastic resins.

This and other objects, as will hereinafter become more readily apparent, has been achieved by counter-currently contacting an SO2-containing exhaust gas with milk of lime, or a suspension of precipitated calcium carbonate, or a mixture of slaked lime and precipitated calcium carbonate, or a mixture of slaked lime, precipitated calcium carbonate and calcium sulte, in an apparatus, such as a venturi scrubber or spray tower, which will absorb the dilute SO2-containing exhaust gas.

In particular, the objects of this invention have been achieved by contacting SO2-containing gas with a suspension of calcium hydroxide, precipitated calcium carbonate, or a mixture thereof. The SO2-containing gas is continuously fed through a series of contact zones while the suspension is fed counter-currently through said series of contact zones so as to contact the gas with the suspension. The first contact zone in the series is maintained at a ternperature of from 50-1001 C. and a pH of 3-6, and each succeeding contact zone is maintained at temperatures of from 50-100 C. and pHs of higher than 6.

BRIEF DESCRIPTION OF THE DRAWING The Figure is a schematic view illustrating one embodiment of the apparatus used for the process of this invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS According to this invention, hexagonal system prismatic calcium sulfite is prepared by counter-currently contacting SO2-containing gas to a suspension of Ca(OH) 2, CaCO3, or a mixture of Ca(O1I-I)2 and CaCOa, or Ca(O1H)2, CaCOS and CaSOs-*l/ZHZOl in an apparatus such as `a venturi scrubber and spraying tower, so that SO2 is absorbed and reacted and a part of the resulting product is recycled.

Referring now to the Figure, the reaction apparatus for reacting SO2 with milk of lime or a suspension of precipitated calcium carbonate or a mixture thereof, comprises venturi scrubbers V1., V2 and V3, and gas-liquid separators R1, R2 and R3. SO2-containing gas is fed through duct 10 through the venturi scrubber and the gas-liquid separator V1. R1 V2- R2 V3- R3 and is driven by blower 12 and exhausted through duct 11.

The milk of lime is fed from the milk of lime tank 4 to a recycling solution tank 5 and is fed at an equivalent concentration to that of the SO2, to the venturi nozzle 3, wherein the milk of lime is reacted with the SO2 which has not been absorbed in the venturi scrubbers V1 and V2. It is then fed through R3 andthe pump 8 to the venturi nozzle 2 and further reacted with SO2 which is not absorbed in V1. It is then fed through the gas-liquid separator R2 and the pump 7 to the venturi nozzle 1 in which the reaction is conducted at a pH of 3 6, preferably about 5. The unreacted slaked lime or calcium carbonate is completely converted to calcium sultite and crystal growth of the resulting calcium sultite is provided by maintaining a slight acidic condition.

A portion of the liquid separated in the gas-liquid separator R1 is recycled through pump (not shown in drawing), and the remainder is fed through pump 9 to the pH adjusting tank 6. In this tank 6, the pH is adjusted to 7 9 and then is filtered and dried to prepare the iinal product.

The SO2-containing gas used for the process of this invention can be any high concentration SO2-containing Vgas such as prepared by combustion of sulfur or iron pyrite. It may alternatively be a dilute, SO2-containing gas, such as boiler exhaust gas. Even though boiler exhaust gas is usually contaminated with dust, the dust can be removed by the use of a suitable dust collector, such as a washing tower, so that contamination difliculties can be avoided.

Conventional milk of lime can be used herein such as that prepared by calcination of limestone in a shaft kiln and then hydrating the resulting quick lime. Hydration is effected preferably at a temperature of higher than 90 C. The concentration of milk of lime used should be from 20 wt. percent to 0.1 wt. percent and preferably from wt. percent to 5 wt. percent, and the milk of lime will be fed to the recycling milk of lime depending on its condition.

The precipitated calcium carbonate can be prepared by reacting CO2 with milk of lime. Preferably, the precipitated calcium carbonate used will have a size of less than 1n which can be prepared by reacting the CO2 gas in the system with the milk of lime.

The reaction of this invention can be shown by the following schematic:

When the SO2-containing gas contains a small amount of CO2 gas, such as will occur in exhaust gas obtained by the partial combustion of H28, the reactions (3) and (4) will predominate. On the other hand, when the SO2-containing gas contains a large amount of CO2 gas, such as is found in boiler exhaust gas, allthe reactions (l), (2), (3) and (4) will occur.

The reaction (l) will occur in an alkaline solution, and the resulting calcium carbonate will be ne particles of less than 1n in size. These are easily converted into CaSO3'1/2H2O by contact with SO2, as shown in the reaction formula (2). In this latter reaction, the solution will become acidic and the CaSO3-1/1H2O will be grown to large particle size hexagonal system prismatic crystals of calcium sulte by reaction (4). The reaction (3) will occur when the solution is alkaline and the resulting CaSO3-1/2H2O will have a fine crystal structure of about 3p. in particle diameter. The reaction (4) will occur when the pH of the solution is acidic. Accordingly, the crystals of CaSO3-1/2H2O can be grown by adding Ca(OH)2 to Ca(HSO3)2, or by removing SO2 from Ca(HSO3)2 so as to shift the equilibrium reaction (4) in the direction of the desired product. The reactions (l) and (3) are neutralization reactions and accordingly these processes are quite simple. Moreover, reactions (2) and (4) can be provided simply by converting the solution to an acidic condition, so that these processes are also quite simple.

ln order to increase the reaction velocity of reaction (2) in each of these four reactions, it is necessary to produce tine particles of calcium carbonate of less than la in diameter. In the reaction of milk of lime and CO2 gas, the velocity of the absorbing reaction will be decreased if the reaction temperature is from 0 C. to 100 C., so that the calcium carbonate produced Within this temperature range will have a particle size diameter of less than La.

The reaction (2) is quite slow as compared with the reaction (3) of SO2 and Ca(OH)2, and accordingly the resulting product will consist mainly of CaSO31/2H2O when SO2 gas and CO2 gas are fed. Accordingly, the CaCO3 resulting from the reaction (l) is quite useful for producing large size .calcium sulfite crystals, since reaction (2) will occur in an acidic condition.

In accordance with this invention, it is most preferable to use the continuous process apparatus as shown in the figure. The apparatus, however, can be horizontally arranged as shown or vertically arranged depending'upon the particular reaction conditions. The product obtained by the subject reaction will usually have particle diameters of S-50/t. The actual particle size can be controlled by 'selecting the conditions within V1, and particularly the pH and the residence time, i.e., recycling ratio. The resulting slurry is neutralized with a small amount of Ca(OH)2 to provide a pH 7-9, and the filtering and drying of the slurry can be conducted by use of such conventional apparatus as ordinary suction lters, rotary kiln driers, etc.

The product CaSO3-1/2H2O having a particle diameter of 5-50,u. can be admixed with a resinous polymer, such as high density polyethylene, in a ratio of 5-90 percent by weight and the mixture is kneaded. The filler may, alternatively, be admixed with the resinous polymer with other llers, such as 3,u. diameter CaSO3'1/2H2O in a ratio of 5 9() percent by weight based on the total weight of filler.

Among the more interesting aspects by this invention, is that since SO2 is easily and continuously absorbed for the reaction, it is a quite effective technique for reducing SO2 air pollution. This invention can be usedfor growing crystals of calcium sulte, whereby particle size control is necessary.

Having generally described the invention, a further understanding can be obtained by reference to certain specific Examples which are provided herein for purposes of illustration only and are not intended to be limiting in any manner unless otherwise specified.

EXAMPLE (Experiment) Temperature of calcium hydroxide dispersion:

V1 98 V2 99 V3 90 Temperature of SO2-containing gas:

Temperature of calcium hydroxide dispersion:

V1 85 V2 70 V3 70 Temperature of SO2-containing gas:

C. V1 v100 Temperature of calcium hydroxide dispersion V2 50 V3 v,50

Temperature of SO2-containing gas:

(Reference) Temperature of calcium hydroxide dispersion:

v1 3o V2A 1 20 5 .i V3 20 :3

Temperature of SO2-containing gas: u C 10,10 ,00 0 100.0 :V1 Y v 36 Sulfur Sulfur i 10 Exhaust gas from heavy burning burning V2 20 oil boiler gas gas V3 20 (A) Condition of A:

Concentration pH Concentration of SO2 (p.p.m.) Young's Type 0I Ca(OH)2 n modulus Softness Processgas (percent) V1 V2 V3 V1 in V2 in V3 1n V3 out (kg/cm!) (mg.) ability 5 5.2 10.0 12.3 9,000 7.000 900 0 28.700 820 Good. 10 5. 5 10. 0 12. 3 9, 000 6,500 800 0 29, 600 810 Do. 15 6.0 8.0 12.3 9,000 6,000 700 0 38,900 1,061 Excellent.

5 4.3 7.8 12.3 79,000 40,000 3,000 0 82,200 986 Good. 10 4.5 9.5 12.3 79,000 35,000 2,500 0 28,100 796 Do. 15 4. 0 11. 1 12. 3 79, 000 50, 000 3,500 0 27, 100 803 Do. 5 3. 0 9. 0 12. 3 150,000 100, 000 5, 000 0 27, 800 932 D0. 10 3. 5 7. 9 12. 3 150, 000 70,000 4,000 0 27, 300 786 Do. 15 3. 9 8. 8 12. 3 150, 000 80, 000 4, 500 0 20, 400 820 Do.

10 8.5 11.5 12.3 9,000 80 0 0 23,500 760 Fair.

(B) Condition of B:

Concenation pH Concentration of SO2 (p.p.m.) Youngs Ca(OH)2 modulus Softness Process- (percent) V1 V2 V3 V1 in V2 in V3 in V3 out (kgJcm) mg.) ability 5 5.2 7.8 12.3 9,000 7, 000 950 0 28,500 724 Good. 10 5. 5 8. 5 12. 3 9, 000 6, 500 870 0 29, 200 832 Do. 15 6.0 8.8 12.3 9,000 6,000 730 0 39,900 1,100 Excellent.

5 4.4 10.2 12.3 79, 000 45,000 3,500 0 37,200 1,000 Good. 10 4. 7 9. 0 12. 3 79, 000 50, 000 4, 500 0 27, 100 928 D0. 15 4. 9 11. 1 12. 3 79, 000 35, 000 2, 500 0 27, 300 865 Do. 5 3. 1 8. 3 12. 3 150,000 110,000 6,500 0 29,000 796 Do. 10 3. 6 9. 5 12. 3 150, 000 000 4, 800 0 28, 200 782 Do. 15 4. 0 9. 9 12. 3 150,000 85, 000 5, 000 0 29, 000 824 Do.

10 9.5 11.8 12.3 79,000 500 0 0 24,000 716 Fair.

(C) Condition of C:

Concentration pH Concentration of S02 (p.p.m.) Young 's Typo of Ca(OH)2 modulus Soitness Processgas (percent) V1 V2 V3 V1 in V2 in V3 in V3 out (kg/cm3) (mg.) ability 5 5.3 7.5 12.3 9,000 6,800 800 0 24,800 728 Good, 10 5. 7 6. 8 12. 3 9, 000 5, 900 780 0 25, 000 786 Do. 15 5.9 7.3 12.3 9,000 4,900 650 0 37,600 1, 046 Excellent.

5 4.3 10.2 12.3 79,000 2,800 0 37,200 989 Good. 10 4. 8 10. 0 12. 3 79, 000 49, 000 3, 900 0 29,000 908 D0. 15 5.0 9. 9 12. 3 79, 000 4, 200 0 28, 600 808 Do. 5 3. 3 7. 8 12. 3 150, 000 95, 000 6, 300 0 27, 200 801 Do. 10 3. 6 8. 2 12. 3 150, 000 89, 000 5, 700 0 26,000 796 D0. 15 3. 8 8. 4 12. 3 150, 000 79, 000 4, 300 0 27, 200 788 IDo.

10 10.0 12.0 12.3 150,000 80,000 5,200 0 24,800 736 Fair.

(D) Condition of D:

Concentration pH Concentration of SO2 (p.p.m.) Youngs Type of Ca(OH)a modulus Soltness Processgas (percent) V1 V2 V3 V1 in V2 in V3 in V3 out (kg/cm!) (mg.) ability I 10 5.2 10.0 12.3 9,000 7,000 900 0 23,800 731 Fair. II 10 4.5 9.5 12.3 79,000 40,000 3,000 0 21,600 718 Do. l III1 10 3.8 8.9 12.3 15,000 100,000 5,000 0 23,000 725 Do,

I.. 10 8.8 10.2 12.3 9,000 7,000 900 0 22.300 745 Fair.

Having nowfully described vthe..invention', it will be apparentto one of ordinary Vskill in the art, that many changes and modifications can be made thereto Without departing from the spirit or scope of the invention. Accordingly, what is claimed as new and desired to be secured by Letters Patent is:

1. In a process for preparing hexagonal system prismatic crystals of calcium sulte by contacting SO2 containing gas with a suspension of calcium hydroxide, precipitated calcium carbonate, orv mixture thereof, the improvement comprising continuously passing an SO2 containing gas through a plurality of contact zones in which the first of said `zones has a temperature of 100 C. and a pH of 3-6, and each succeeding Contact zone has a'temperature of 50-100 C. and a pH of higher than 6; and

simultaneously continuously passing said suspension through said plurality of contact zones, countercurrently to the direction of flow of said SO2- containing gas, wherein ne particles of calcium sulte, having an average diameter less than 3p are produced in the last contact zone and hexagonal prismatic -crystals of calcium sulite having a particle size of greater than 5p. in average diameter are grown in the first contact zone.

` 2. The process according to Claim 1, wherein said suspension is a milk of lime.

3- The..prosssacgtdinato ClimLwherenaif S02- containing 'gasis'an exhaust gas 'containing SO2?" 4. The process according to Claim 1, wherein said SO2- .containingV 'gas isv produced by combustion of sulfur',` or iron .pyrite' W v 5. The process according to Claim 2, wherein the concentration of said milk of ,lime/:in saidlast contact zone is from about 20 wt. percent to 0.2 wt. percent.

6. The process according to Claim 2, wherein said SO2- containing gas contains CO2 gas so that the precipitated calcium carbonate is converted to milk of lime.

7. The process according to Claim 1, wherein fine particles of calcium sulite are produced in the second contact zone to the last contact zone, and the product calcium sulte particles are grown inthe first contact 2mm Y 8. The process according to Claim 1, wherein said suspension is fed with said SO2-containing gas in each contact zone, while said suspension is fed vfrom the last contact zone to the first contact zone and said SO2-containing gas is fed from the irst contact zone to the last contact zone.

References Cited UNITED STATES PATENTS 2,210,405 8/1940 Haywood 423-512 1,984,188 12/1934 Haywood 423-512 X EARL C. THOMAS, Primary Examiner 

